The present invention relates to digital communications and, in particular, to the demodulation of adjacent channel signals in digital communications systems.
A primary consideration in any digital communications system is the channel bandwidth required to transmit information. Generally, digital systems are designed to utilize channel bandwidth as efficiently as possible. For example, in systems utilizing frequency division multiplexing, maximum spectral efficiency is obtained by spacing frequency channels very close to one another in an available spectrum.
Minimum carrier spacing is limited in practice, however, by adjacent channel interference. As shown in FIG. 1, adjacent channel interference is defined as the interference resulting when carrier frequencies are spaced close enough to one another that information signals modulated on the corresponding carriers overlap in the frequency spectrum. In FIG. 1, first and second modulated signals s.sub.1, s.sub.2 having first and second bandwidths B1, B2 are transmitted using first and second carrier frequencies f.sub.1, f.sub.2, respectively. The carrier, or channel, spacing between the first and second carrier frequencies f.sub.1, f.sub.2 is such that the first and second modulated signals s.sub.1, s.sub.2 overlap in a region of interference INT.
In practice, the minimum allowable carrier spacing is a function of the bandwidths of the information signals, the practical limitations associated with receiver filtering, and the signal modulation and coding schemes used. Any design improvement providing increased suppression of adjacent channel interference can be used advantageously to increase system capacity, relax coding and modulation design requirements, or improve signal quality.
In conventional systems, adjacent channel interference is suppressed in a number of ways. For example, in certain cellular radio systems, adjacent channel interference is avoided through channel allocation schemes in which channels immediately adjacent to one another in frequency are assigned to different spacial cells. Consequently, physical separation reduces mutual interference between adjacent channels. Such a system is described, for example, in IEEE Transactions on Vehicular Technology, Vol. 43, November 1994, S. Golestanch, "The effect of ACI on the capacity of FDMA cellular systems", which is incorporated herein by reference. In other communications systems (e.g., satellite and land mobile radio systems), however, suppression of adjacent channel interference by physical separation of adjacent channels may not be possible.
An alternative conventional approach is described in S. Sampei and M. Yokoyama, "Rejection Method of Adjacent Channel Interference for Digital Land Mobile Communications," The Transactions of the IECE of Japan, Vol. E 69, No. 5, pp. 578-580, May 1986, which is incorporated herein by reference. The cited method teaches that, during demodulation of a given carrier signal, a bandpass filter centered at an adjacent carrier is used to extract an adjacent channel signal (ACS) at the adjacent carrier. The extracted signal is then used to estimate the adjacent channel signal envelope and carrier and to coherently detect the adjacent channel signal. The detected adjacent channel signal is then waveform shaped, and the estimated adjacent channel carrier and envelope are impressed on the resulting signal. Ideally, the described process provides a reconstructed adjacent channel signal at its carrier frequency. The reconstructed signal can then be passed through a bandpass filter centered at the carrier of interest and subtracted from the received signal to remove the adjacent channel interference.
Such an approach has several limitations, however. For example, analog signal processing using filters and mixers adds undesirable cost and size to a radio receiver, and since the analog components vary with the manufacturing process, such receivers provide a relatively unpredictable range of performance. Additionally, subtracting a signal at radio frequency requires highly accurate carrier reconstruction and time alignment, as an error as small as half a cycle at radio frequency can cause the adjacent channel signal to double rather than diminish. Furthermore, such use of the adjacent channel carrier (phase and frequency) and envelope (amplitude) implicitly assumes that the radio channels are not dispersive. However, in many practical wireless systems (e.g., D-AMPS and GSM), the symbol rate is sufficiently high that the radio transmission medium must be modeled to include time dispersion which gives rise to signal echoes. Thus, the proposed technique is not always practical for use in many present day applications.
According to another conventional approach, demodulation parameters such as linear or decision feedback equalization filter coefficients are adapted to minimize noise and adjacent channel interference together. See, for example, IEEE Transactions on Communications, Vol. COM-42, December 1994, B. R. Petersen, "Suppression of Adjacent-Channel, Cochannel, and Intersymbol Interference by Equalizers and Linear Combiners". Alternatively, spectrally efficient continuous phase modulation (CPM) techniques can be used to reduce the effects of adjacent channel interference. See, for example, IEEE Transactions on Communications, Vol. COM-34, November 1986, V. K. Varma and S. C. Gupta, "Performance of partial response CPM in the presence of ACI and Gaussian noise".
As noted above, however, minimizing or avoiding adjacent channel interference using the above described systems provides only marginal improvement with respect to spectral efficiency, and current suppression mechanisms are inadequate for broad applications. Thus, there is a need for improved methods and apparatus for significantly reducing the impact of adjacent channel interference.